The below-listed prior art patents are noted as being pertinent with respect to the general problem, but most of which employ a vacuum system on the driving or rotating roll: U.S. Pat. Nos. 2,679,572; 2,826,827; 3,097,933; 3,118,743; 3,122,295; 3,151,796; 3,258,184; 3,286,895; 3,327,916; 3,366,298; 3,405,855; 3,420,424; 3,465,320; 3,561,133; 3,688,336; 3,782,003; and 3,854,222.
In addition, the following references are listed as being pertinent in the general performance of heat transfer rolls and particularly chill-rolls important to "ink setting" published in (1) "An Investigation of Self-Acting Bearings," published in the Journal of Basic Engineering, December 1965, pages 837 through 846; (2) "Fluid Effects Associated with Web Handing" by Kenneth L. Knox and Thomas L. Sweeney, published at Ohio State College Ind. Eng. Chem. Process Des. Development, Volume 10, No. 21971, page 201; and "Graphic Arts Monthly"--June 1980, pages 76 through 82.
In foil bearings, the air layer between the roll and web is desirable because the web then floats over the rolls with a small amount of friction between the rotating or stationary roll (bearing) and the web. However, in heat transfer rolls, this air layer is undesirable because of resistance of the insulating qualities of the air layer trapped between the web and heat transfer roll surface, because it reduces addition or removal of heat. Its effect should be reduced if possible.
The above-referenced published literature indicates that an air layer forms under the web at the nip of the roll where the web touches the roll surface or, in other words, where the boundary layers, which are attached to the heat transfer roll surface and to the web come together and meet. This thickness h.sub.o of the air layer trapped at the nip of the web and roll can be calculated. It is less than the sum of each individual boundary layer meeting at the nip. This is expressed by the following formula EQU h.sub.o =0.65R(6.mu.u)/.pi.).sup.2/3
for a non-rotating roll.
If the roll rotates with a peripheral velocity equal to and in the same direction as the web, the factor 6 in the above equation is replaced by a factor of 12. In all other equations for h.sub.o listed in reference 1, the factor .mu. appears also.
.mu.=viscosity of entrapped air PA1 u=film speed (Ref. 1) PA1 .pi.=tension of web PA1 .delta.=boundary layer thickness PA1 S=fluid density PA1 V=web velocity
For dimensions of the parameters in the above equation, see Reference 1.
Tests shown on page 841 of Ref. 1 show a smaller thickness h.sub.o than calculated as above.
It is generally concluded, backed up by calculations and tests, that the air layer at the nip is much smaller than the boundary layers carried along by roll and web meeting at the nip.
Also, the boundary layer at the exit quickly establishes itself after the web leaves the roll and the web may also oscillate (Ref. 1 FIG. 15).
Boundary layer thickness, Ref. 2, page 202, equation (4) gives the boundary layer thickness EQU .delta.=6.37(.mu.X/SV).sup.1/2
where X is the distance from the start of the boundary layer that develops on a moving, continuous flat film in a stagnant infinite fluid
(for dimensions, see Ref. 2).
Factors such as machine vibrations, film flutter, gravity, centrifugal force acting on the web where it changes direction at the nip and gauge variations have been neglected (page 205 of Ref. 2).
Looking particularly at the use of a chill roll in an offset print operation where the web is heated, the boundary layer attached to a 300.degree. F. web may be higher than 300.degree. F. because it may come out of a 500.degree. F. air dryer. The boundary layer attached to the first chill roll, after the web leaves the first chill roll, may also be higher than room temperature. It's temperature will be between that of the chill roll surface and that of the web). When these two boundary layers meet at the nip of the roll, the temperature of the air layer at the nip will be much higher than that of the surrounding air.
The general purpose of this invention then is to reduce the effect of the air layer at the nip of the roll. Since boundary layers in general and air layer at the nip both have the viscosity .mu. in the equation with an exponent of 2/3 and 1/2, this effect can be reduced by reducing the viscosity .mu. or by changing other factors affecting .mu..
Chemical Engineers Handbook Perry 3rd edition, page 371, FIG. 17 gives viscosity in centipoises of gases at 1 atm. For instance, lowering the temperature of the air layer at the nip from 400.degree. F. to 80.degree. F. lowers viscosity .mu. of air from 0.025 to 0.018 or to 18/25=0.72 to what it was before. Air layer at the nip and its thermal resistance should be reduced to 0.72.sup.2/3 =0.88 or by 12 percent. Assuming a previous overall thermal resistance between the web and heat transfer roll of 0.05 to 0.08, this would now be reduced by 0.12 X (0.05 to 0.08) or by 0.006 to 0.0096. Hence, with viscosity being the most influential component in the overall heat transfer resistance of approximately 0.07, between chill roll and web improvements in overall heat transfer may be about 10 percent or more by changing or reducing viscosity.